You can actually have advantages by levelling up your classes somewhat together, as you sometimes find hard to find foes for the Hunt Log and they can benefit both classes, but many prefer to level up to 30 and then come back and do the secondary class to 10 or 15, i found my secondary class so fun i plan to keep going with it. Enjoy no matter what, no rush.... so much to see and we only see it for the first time once.